In order to replace a defective active component by an identical standby component without an objectionable discontinuity in signal transmission, it is essential that the two components by precisely synchronized by receiving recurrent timing signals strictly in step with each other. These timing signals are usually derived from a master clock by frequency division in a step-down circuit such as a pulse counter forming part of a time-base unit. The use of a single time-base unit for the operating and standby components entails the risk that failure of this unit may disable both components controlled by it. This risk is, of course, aggravated when there are more than two components to which timing pulses must be supplied in synchronism, such as a principal component, a standby component and a reserve component adapted to be substituted for one another.
It is, therefore, convenient to duplicate not only the operating component but also the associated time-base unit. However, if one of two such units falls out of step with the other, it may be difficult or impossible to ascertain instantaneously which of these units is keeping correct time. Thus, even with only two controlled components there should be at least three time-base units which are normally synchronized with one another so that loss of synchronism is a single unit will readily reveal itself.
Conventional chronometric systems of this description suffer from a variety of drawbacks, including complex and correspondingly expensive circuitry, limited reliability, lenghty start-up or recovery periods, need for very precise and thus costly signal generators, and lack of exact synchronism between the trains of output pulses generated by the serveral time-base units.
Another problem arising in such a system is the need for an instantaneous switchover from a master oscillator, generating the original clock pulses for all time-base units, and a standby oscillator in the event of failure of the former.